Just because some train lengths have been increasing anyway, doesn't mean that there wouldn't have been more without the current system.
Well, I am of the opinion that it does mean exactly that, ie that variable track access costs are not a deciding factor.
I have been involved in some way or other with several* train lengthening projects, and I can’t think of any point, in any of them, where there was a discussion that said “we’d like to do this, but variable track access is an issue”.
By far the greater costs in the mix for train lengthening are the costs of changing the infrastrcuture (longer platfroms, longer depots and sidings, power supply; sometimes moving signals, points, even neutral sections), additional train maintenance and cleaning, additional fuel / traction electricity, and of course the cost of the vehicles themselves.
*at least 10, probably more.